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ntroduction

The notes below provide identify the factors which affect the properties of timber. More

detailed information on wood types and properties is provided on webpage Wood

The timber used for construction, carpentry, joinery etch is obtained from the trunks of trees.

This part of the tree includes features required to structurally support the tree its branches and

leaves and withstand natural forces such as wind. The truck also includes a cellular structure totransport moisture containing minerals and nutrients, sap between the roots and the upper

branches and leaves.

The trunk is composed of long tubular cells made up of cellulose which are bonded together by

lignin. Most of these tubular cells run along the trunk and result in the grain structure of the

wood. some cells run radially out from the centre of the trunk and these are identified as rays.In temperate countries, trees produce a new layer each growing season under the bark and this

results in the annular rings. In tropical regions which have continuous growth the trunk

includes and essentially uniform section.

The central, normally darker, area of the trunk is called the heartwood and the outer area is calledthe sapwood. The heartwood performs a mechanical support function and stiffness to the tree.

The sapwood transports the fluids along the trunk. The heartwood is generally stronger and

more durable .

Density...The density of is a very important factor affecting the strength of wood. The densest woods are

generally the strongest. The density is also related to the stiffness and hardness e.g both theseproperties increase as the timber density increases.

Softwoods...Softwoods are generally evergreens (coniferous) with needle-like leaves. They have the

following characteristic features

They grow relatively quickley and trees softwood trees can be felled after 30 yearsThey have, generally, poor durability qualities unless treated with preservatives.

http://www.roymech.co.uk/Useful_Tables/Matter/Wood.htmlhttp://www.roymech.co.uk/Useful_Tables/Matter/Wood.html

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They are readily available and comparitively cheap

Hardwoods...Hardwoods are generally broad-leaved (deciduous) that lose their leaves at the end of each

growing season. The cell structure of hardwoods is more complex than softwoods with thick

walled cells called fibres providing the structural support and thin walled cells called vesselsproviding the meddium for food condution along the trees. Hardwoods have the followingcharacteristics affecting their use.

The grow relatively slowly ,resulting in denser, stronger timbers. These tress

take over 100 years to mature.

They have, generally, good durability qualities with less need for preservatives.

They are obtained from tropical areas and are comparatively expensive. .

Moisture ContentTimber has high moisture content when first processed from the tree. The moisture content is

reduced prior to the timber being used as a structural material. The drying off timber can beachieved using either of two methods

1) Air seasoning ...The timber is stacked in open sided weatherprrof containers

is such the air can freely circulated to all surfaces of the timber to promote drying.

This process can take considerable time and much space is required.

2) Kiln Drying..The timber is dried out in heated , ventilated and dehumidified

ovens. This process requires special equipment and requires high energy energy

use. However the process is controlled and rapid.

Because of the nature of timber poor control of the drying process can result in a range of defectsincluding bowing, cupping, twisting and cracking.

The moisture content of wood w() is defined using the equation

w = (m1- m2) / m2

w = moisture content expressed as a percentage.

m1mass of test piece before drying (g)

m2mass of test piece after drying (g)

Directional Properties / AnisotropyThe properties of timber are different in different directions. The elastic modulus and strength

of wood is much greater in a direction along the grain than across the grain. If the grain issloping at an angle to the direction of loading this can also have a marked effect of the strength of

the timber.

Stress/Strain relationshipAt low stress levels the strain is proportional to the strain. The strain is greater for a given stressat high moisture contents. However timber does have significant creep characteristics i.e over

time at high constant stress values the strain will increase. In calculating strength of structures

the duration of the loading is one of the factors to be considered.

Thermal Properties

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Timber is a poor conductor of heat. The denser timbers are better conductors than the lightertimbers. Timber expands when heated but this is offset by the reduction in section resulting from

the loss of moisture.

Fire ResistanceTimber compares favourably with other construction materials in terms of fire resistance.Small timber section may ignite easily and reduce to ash. However larger sections have

significant resistance to fire because the surfaces initially in contact with the flames burn to

charcoal which resists the further spread of the flame.

DurabilityTimber does not corrode. It is however subject to weathering, chemical attack, and fungal

attack. The durability of timber varies depending on the wood species If timber is kept dry or

is continuously immersed in fresh water then decay shoud not be a problem. Timber immersedin seawater is at high risk of damage resulting from attack by molluscs.

There are a number of preservative treatments available to prevent the deterioration of timber

resulting from fungi, insects or molluscs. The types of wood presevative treatments available

are briefly described bel

1) Tar oil penetration - Creosote is a a typical tar oil presevative. This group is

used generally for external systems such as telegraph poles , fences, railway

sleepers, timber enclosures (garden sheds) etc.

2) Organic solvents.. These are widely used and are applied using simple

techniques such a brushing spraying or dipping.

3) Water borne solvents are the most widely used for industrial applications and

are normally applied into the timber under pressure.